Conventionally, the vibration screen has been provided with such a vibration generator as to generate vibration by rotating in mutually reverse directions the two rotating shafts equipped with eccentric weights arranged in parallel (e.g. refer to patent literature 1).
With the vibration generator of this type, the two rotating shafts are rotated mutually in reverse directions by linking them using gears as a means to rotate them and conveying the rotary drive force of one motor to each of the rotating shafts, thus rotating them mutually reverse directions in a synchronized way.
On the other hand, there exists another type not requiring the gears (this is called “gearless mechanism” in this specification) by arranging two motors so as to drive and rotate the two rotating shafts independently. In case of the vibration generator using this gearless mechanism, it is advantageous in reducing power consumption because there is no energy loss due to the gears and in checking noise during operation, but the following disadvantages exist.                (1) In case of two motors (total capacity kW), the starting torque theoretically requires about 20% larger in comparison with the starting torque (capacity of a motor) required to drive the vibration generator using gears. Since this type of machine does not normally exceed the rated current during load operation, it is normally selected with the starting torque.        (2) For this reason, the load rate against the capacity of motor becomes lower than the vibration generator using gears, and it leads to an increase in power consumption from the viewpoint of characteristics of the motor (efficiency is low in the use state with a low load rate).        (3) Since two motors are installed, it is difficult to arrange the motors on one side, and therefore, they are arranged on both sides of the vibration screen. In this case, space for arranging the motors becomes large, and therefore, the motors and vibration generator are integrated and installed to save the space, and they are generally vibrated together with the vibration screen, but for this reason, however, the weight of the vibration generator increases, finally leading to an increase in power consumption.        
By the way, this applicant, considering the problems of the vibration screen using the vibration generator applying the above conventional gearless mechanism, proposed a vibration generator capable of reducing the motor capacity and peak power consumption required (see Patent Literature 2).